Speciation is the evolutionary process by which new biological
species arise. The biologist
Orator F. Cook
seems to have been the first to coin the term 'speciation' for the
splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic
evolution' occurring within lineages.[1][2] Whether
genetic drift is
a minor or major contributor to speciation is the subject of much
ongoing discussion. There are four geographic modes of speciation
in nature, based on the extent to which speciating populations are
geographically isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be
induced artificially, through animal husbandry or laboratory
experiments. Observed examples of each kind of speciation are
provided throughout.[3]

Natural
speciation

All forms of natural speciation have taken place over the course
of evolution; however it still remains a subject of debate as to
the relative importance of each mechanism in driving
biodiversity.[4]

The three-spined stickleback (Gasterosteus
aculeatus)

One example of natural speciation is the diversity of the three-spined stickleback, a marine fish that,
after the last ice age, has undergone speciation into new freshwater colonies in
isolated lakes and streams. Over an estimated 10,000 generations,
the sticklebacks show structural differences that are greater than
those seen between different genera of fish including variations in
fins, changes in the number or size of their bony plates, variable
jaw structure, and color differences.[5]

There is debate as to the rate at which speciation events occur
over geologic time. While some evolutionary biologists claim that
speciation events have remained relatively constant over time, some
palaeontologists such as Niles Eldredge
and Stephen
Jay Gould have argued that species usually remain unchanged
over long stretches of time, and that speciation occurs only over
relatively brief intervals, a view known as punctuated equilibrium.

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Allopatric

During allopatric speciation, a population splits into two
geographically isolated allopatric populations (for example, by habitat
fragmentation due to geographical change such as mountain building or social change such as
emigration). The
isolated populations then undergo genotypic and/or phenotypic
divergence as they (a) become subjected to dissimilar selective pressures or (b)
they independently undergo genetic drift. When the populations come
back into contact, they have evolved such that they are
reproductively isolated and are no longer capable of exchanging
genes.

Observed instances

Island genetics, the tendency of small,
isolated genetic pools to produce unusual traits, has been observed
in many circumstances, including insular dwarfism and the radical
changes among certain famous island chains, for example on Komodo. The Galápagos islands are particularly famous
for their influence on Charles Darwin. During his five weeks
there he heard that Galápagos tortoises could be
identified by island, and noticed that Mockingbirds differed from one island to
another, but it was only nine months later that he reflected that
such facts could show that species were changeable. When he
returned to England, his speculation on evolution deepened after experts informed him
that these were separate species, not just varieties, and famously
that other differing Galápagos birds were all species of finches.
Though the finches were less important for Darwin, more recent
research has shown the birds now known as Darwin's
finches to be a classic case of adaptive evolutionary
radiation.[6]

Peripatric

In peripatric speciation, new species are formed in isolated,
small peripheral populations that are prevented from exchanging
genes with the main population. It is related to the concept of a
founder
effect, since small populations often undergo bottlenecks. Genetic drift is
often proposed to play a significant role in peripatric
speciation.

Reproductive isolation occurs in populations of Drosophila subject
to population bottlenecking

The London Underground mosquito
is a variant of the mosquito Culex pipiens that entered in
the London
Underground in the nineteenth century. Evidence for its
speciation include genetic divergence, behavioral differences, and
difficulty in mating.[7]

Parapatric

In parapatric speciation, the zones of two diverging populations
are separate but do overlap. There is only partial separation
afforded by geography, so individuals of each species may come in
contact or cross the barrier from time to time, but reduced fitness
of the heterozygote leads
to selection for behaviours or mechanisms that prevent breeding between the two
species.

Ecologists refer to parapatric and peripatric speciation in
terms of ecological niches. A niche must be available in order for a
new species to be successful.

Sympatric

In sympatric speciation, species diverge while inhabiting the
same place. Often cited examples of sympatric speciation are found
in insects that become dependent on different host plants in
the same area. However, the existence of sympatric speciation as a
mechanism of speciation is still hotly contested. People have
argued that the evidences of sympatric speciation are in fact
examples of micro-allopatric, or heteropatric speciation. The
most widely accepted example of sympatric speciation is that of the
cichlids of Lake Nabugabo in East Africa, which is
thought to be due to sexual selection. Sympatric
speciation refers to the formation of two or more descendant
species from a single ancestral species all occupying the same
geographic location.

Until recently, there has a been a dearth of hard evidence that
supports this form of speciation, with a general feeling that
interbreeding would soon eliminate any genetic differences that
might appear. But there has been at least one recent study that
suggests that sympatric speciation has occurred in Tennessee cave
salamanders.[8]

The three-spined sticklebacks, freshwater fishes, that have been
studied by Dolph Schluter (who received his Ph.D. for his work on
Darwin's finches with Peter Grant) and his current colleagues in
British Columbia, were once thought to provide an intriguing
example best explained by sympatric speciation. Schluter and
colleagues have found:

Two different species of three-spined sticklebacks in each of
five different lakes.

a large benthic species with a large mouth that
feeds on large prey in the littoral zone

a smaller limnetic species — with a smaller mouth —
that feeds on the small plankton in open water.

DNA analysis indicates that each lake was colonized
independently, presumably by a marine ancestor, after the last ice
age.

DNA analysis also shows that the two species in each lake are
more closely related to each other than they are to any of the
species in the other lakes.

Nevertheless, the two species in each lake are reproductively
isolated; neither mates with the other.

However, aquarium tests showed that

the benthic species from one lake will spawn with the benthic
species from the other lakes and

likewise the limnetic species from the different lakes will
spawn with each other.

These benthic and limnetic species even display their mating
preferences when presented with sticklebacks from Japanese lakes;
that is, a Canadian benthic prefers a Japanese benthic over its
close limnetic cousin from its own lake.

Their conclusion: in each lake, what began as a single
population faced such competition for limited resources that

disruptive selection — competition favoring fishes at either
extreme of body size and mouth size over those nearer the mean —
coupled with

assortative mating — each size preferred mates like it -
favored a divergence into two subpopulations exploiting different
food in different parts of the lake.

The fact that this pattern of speciation occurred the same way
on three separate occasions suggests strongly that ecological
factors in a sympatric population can cause speciation.

However, the DNA evidence cited above is from mitochondrial
DNA (mtDNA), which can often move easily between closely
related species ("introgression") when they hybridize. A
more recent study,[9] using
genetic markers from the nuclear genome, shows that limnetic forms
in different lakes are more closely related to each other (and to
marine lineages) than to benthic forms in the same lake. The
threespine stickleback is now considered an example of "double
invasion" (a form of allopatric speciation) in which repeated
invasions of marine forms have subsequently differentiated into
benthic and limnetic forms. The threesspine stickleback provides an
example of how molecular biogeographic studies that rely solely on
mtDNA can be misleading, and that consideration of the genealogical
history of alleles from multiple unlinked markers (i.e. nuclear
genes) is necessary to infer speciation histories.

Sympatric speciation driven by ecological factors may also
account for the extraordinary diversity of crustaceans living in
the depths of Siberia's Lake Baikal.

Speciation via
polyploidization

Polyploidy is a
mechanism often attributed to causing some speciation events in sympatry.
Not all polyploids are reproductively isolated from their parental
plants, so an increase in chromosome number may not result in the
complete cessation of gene flow
between the incipient polyploids and their parental diploids (see
also hybrid
speciation).

Polyploidy is observed in many species of both plants and
animals. In fact, it has been proposed that all of the existing
plants and most of the animals are polyploids or have undergone an
event of polyploidization in their evolutionary history. However,
reproduction is often by parthenogenesis since polyploid animals
are often sterile. Rare instances of polyploid mammals are known,
but most often result in prenatal death.

Hawthorn
fly

One example of evolution at work is the case of the hawthorn
fly, Rhagoletis
pomonella, also known as the apple maggot fly, which
appears to be undergoing sympatric speciation.[10]
Different populations of hawthorn fly feed on different fruits. A
distinct population emerged in North America in the 19th century
some time after apples, a
non-native species, were introduced. This apple-feeding population
normally feeds only on apples and not on the historically preferred
fruit of hawthorns. The
current hawthorn feeding population does not normally feed on
apples. Some evidence, such as the fact that six out of thirteen allozyme loci are different,
that hawthorn flies mature later in the season and take longer to
mature than apple flies; and that there is little evidence of
interbreeding (researchers have documented a 4-6% hybridization
rate) suggests that sympatric speciation is occurring. The
emergence of the new hawthorn fly is an example of evolution in
progress.[11]

Speciation via hybrid
formation

Reinforcement (Wallace
effect)

Reinforcement is the process by which natural selection
increases reproductive isolation.[12] It
may occur after two populations of the same species are separated
and then come back into contact. If their reproductive isolation
was complete, then they will have already developed into two
separate incompatible species. If their reproductive isolation is
incomplete, then further mating between the populations will
produce hybrids, which may or may not be
fertile. If the hybrids are infertile, or fertile but less fit than
their ancestors, then there will be no further reproductive
isolation and speciation has essentially occurred (e.g., as in horses and donkeys.) The reasoning behind this is that if
the parents of the hybrid offspring each have naturally selected
traits for their own certain environments, the hybrid offspring
will bear traits from both, therefore would not fit either
ecological niche as well as the
parents did. The low fitness of the hybrids would cause selection
to favor assortative mating, which would
control hybridization. This is sometimes called the Wallace effect
after the evolutionary biologist Alfred Russel Wallace who
suggested in the late 19th century that it might be an important
factor in speciation.[13] If
the hybrid offspring are more fit than their ancestors, then the
populations will merge back into the same species within the area
they are in contact.

Reinforcement is required for both parapatric and sympatric
speciation. Without reinforcement, the geographic area of contact
between different forms of the same species, called their "hybrid
zone," will not develop into a boundary between the different
species. Hybrid zones are regions where diverged populations meet
and interbreed. Hybrid offspring are very common in these regions,
which are usually created by diverged species coming into secondary
contact. Without reinforcement the two species would have
uncontrollable inbreeding. Reinforcement may be induced in
artificial selection experiments as described below.

Artificial
speciation

New species have been created by domesticated animal
husbandry, but the initial dates and methods of the initiation
of such species are not clear. For example, domestic sheep
were created by hybridisation, and no longer produce viable
offspring with Ovis
orientalis, one species from which they are descended.[14]
Domestic cattle, on the other
hand, can be considered the same species as several varieties of
wild ox, gaur, yak, etc., as
they readily produce fertile offspring with them.[15]

The best-documented creations of new species in the laboratory
were performed in the late 1980s. William Rice and G.W. Salt bred
fruit flies, Drosophila melanogaster,
using a maze with three different choices of habitat such as
light/dark and wet/dry. Each generation was placed into the maze,
and the groups of flies that came out of two of the eight exits
were set apart to breed with each other in their respective groups.
After thirty-five generations, the two groups and their offspring
were isolated reproductively because of their strong habitat
preferences: they mated only within the areas they preferred, and
so did not mate with flies that preferred the other areas.[16] The
history of such attempts is described in Rice and Hostert
(1993).[17]

Dodd's experiment has been easy for many others to replicate,
including with other kinds of fruit flies and foods.[19]

Genetics

Few speciation genes have been found. They usually involve the
reinforcement process of late stages of speciation. In 2008 a
speciation gene causing reproductive isolation was reported.[20] It
causes hybrid sterility between related subspecies.

Hybrid
speciation

Hybridization between two different species sometimes leads to a
distinct phenotype. This
phenotype can also be fitter than the parental lineage and as such
natural
selection may then favor these individuals. Eventually, if reproductive isolation is
achieved, it may lead to a separate species. However, reproductive
isolation between hybrids and their parents is particularly
difficult to achieve and thus hybrid speciation is considered an
extremely rare event. The Mariana Mallard arose from hybrid
speciation.

Hybridization without change in chromosome number is called homoploid hybrid
speciation. It is considered very rare but has been shown in Heliconiusbutterflies[21]
and sunflowers. Polyploid speciation, which involves
changes in chromosome number, is a more common phenomenon,
especially in plant species.

Gene transposition as a
cause

Theodosius Dobzhansky, who
studied fruit flies in the early days of genetic
research in 1930s, speculated that parts of chromosomes that switch
from one location to another might cause a species to split into
two different species. He mapped out how it might be possible for
sections of chromosomes to relocate themselves in a genome. Those mobile sections can
cause sterility in inter-species hybrids, which can act as a speciation
pressure. In theory, his idea was sound, but scientists long
debated whether it actually happened in nature. Eventually a
competing theory involving the gradual accumulation of mutations
was shown to occur in nature so often that geneticists largely
dismissed the moving gene hypothesis.[22]

However, 2006 research shows that jumping of a gene from one
chromosome to another can contribute to the birth of new
species.[23] This
validates the reproductive isolation mechanism, a key component of
speciation.[24]

Interspersed repeats

Interspersed repetitive DNA
sequences function as isolating mechanisms. These
repeats protect newly evolving gene sequences from being overwritten by gene
conversion, due to the creation of non-homologies between otherwise
homologous DNA sequences. The
non-homologies create barriers to gene conversion. This barrier allows
nascent novel genes to evolve without being overwritten by the progenitors of these genes. This uncoupling
allows the evolution of new genes, both within gene families and also
allelic forms of a gene. The
importance is that this allows the splitting of a gene pool without requiring
physical isolation of the organisms harboring those gene
sequences.

Human
speciation

Humans have genetic similarities with chimpanzees and gorillas,
suggesting common ancestors. Analysis of genetic drift and
recombination using a Markov model suggests humans and
chimpanzees speciated apart 4.1 million years ago.[25]

^J.M. Baker (2005). "Adaptive
speciation: The role of natural selection in mechanisms of
geographic and non-geographic speciation". Studies in History
and Philosophy of Biological and Biomedical Sciences36: 303–326. doi:10.1016/j.shpsc.2005.03.005.available online

^E.B. TAYLOR, J.D. McPHAIL (2000).
"Historical contingency and determinism interact to prime
speciation in sticklebacks". Proceedings of the Royal Society
of London Series B267:
2375–2384.[1]available
online

^
Hiendleder S., et al. (2002) "Molecular analysis of wild
and domestic sheep questions current nomenclature and provides
evidence for domestication from two different subspecies"
Proceedings of the Royal Society B: Biological Sciences269:893-904

available online - ScienceDirect - Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences : Adaptive speciation: the role of natural selection in mechanisms of geographic and non-geographic speciation